Possible Carbonate Minerals Within an Unnamed Gullied Crater in

46th Lunar and Planetary Science Conference (2015)
BASIN, MARS. L. K. Korn and M. S. Gilmore, Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT, ([email protected]).
Introduction: Carbonate minerals have been found
in the Martian dust [1], detected in situ at the polar and
Gusev Crater landing sites [2,3], and found remotely
within the Nili Fossae Region [4], in the Leighton [5],
Jezero [6], and McLaughlin [7] craters, in Libya Montes [8,9], in Capri Chasma [10], in the Tyrrhena Terra
Region [11], and in the Huygens Basin [12]. Here we
describe carbonates in an unnamed crater within the
Eridania Basin, part of Terra Sirenum.
Geologic Setting: The unnamed ~60km diameter
crater excavates Noachian-aged units [13] at ~178°W,
39°S, and is cut by fossae related to Tharsis [e.g., 14].
The crater contains glacial deposits, gullies and chaotic
terrain. Carbonates are associated with multiple generations of gullies and depositional fans on the northeast
rim of the crater as well as local deposits on the crater
floor [15] (Fig. 1). The carbonate signature is also associated with high albedo materials in the crater wall,
talus deposits, and boulders weathering from these
deposits. This wall rock has been exposed by the formation and headward retreat of some of the gullies.
Carbonate found within the depositional fan below is
believed to have been transported via the gully’s channels. Thus, the carbonates are localized Noachian-aged
deposits that have been exposed by the formation of
the crater and modified by gullies.
Methods: Long and short wavelength data (0.354.0µm) for two full-resolution CRISM images (Fig. 1),
were processed [16] in ENVI using CAT 7.2.1
[17,18], including radiometric and atmospheric correction and noise reduction [19]. Spectral summary products [16,20] were subsequently generated for each image.
Regions of interest (ROI) were located based on a
RGB summary product combination of BDI1000IR,
D2300 [20] and BD2500 [21], which looks for 1.0µm,
2.3µm, and 2.5µm features, respectively. ROIs containing 10s to 100s of pixels were chosen based on the
brightest pixels within individual summary products.
Derived spectra were ratioed with relatively spectrally
neutral areas of the same width and ROI size (preferably located within the same data column). Spectra with
the same shape and absorption minima were averaged
when applicable.
Carbonate Composition: Carbonate minerals
have prominent overtone and combination absorptions
within the VNIR at 2.35µm, 2.55µm, 3.4µm, and
3.9µm [22,23]. The latter two absorptions are not always present in carbonates [4]. The positions of the
2.3µm and 2.5µm minima shift depending on the size
of the associated cations [22].
Ratioed and averaged spectra from the crater contain 1.0µm, 1.9µm, 2.3µm, 2.5µm, and sometimes
3.9µm features, indicating that carbonate minerals are
present (Fig. 1). A 1.4µm feature is sometimes visible
in continuum removal. The 1.4µm and 1.9µm features
are attributed to water [24], while 1.0µm corresponds
to ferrous iron [22]. Both water and iron may be mineral components or impurities. High calcium-pyroxene
(HCP) minerals with ~1.0µm and ~2.0µm absorptions
[24] are also detected within the crater (Fig. 1).
Figure 1: The top image contains HiRISE BD_013222_1406, CTX
B09_013222_1406, and CRISM data; boxes indicate mineral locations.
Select spectra from the crater are shown in Graph (A) with their corresponding library mineral [25, 26] matches in Graph (B). Apparent
absorptions are indicated by grey lines. Graphs (C) and (D) show a
range of specific carbonate absorptions in continuum removal. Keys in
(C) and (D) match (A) and (B), respectively. Pink spectra are HCP
minerals outside of the crater (A) and library Clinopyroxene/augiteLAPP95 (B). Yellow spectra also represent ankerite.
46th Lunar and Planetary Science Conference (2015)
The locations of the minima for the 2.3µm and
2.5µm features, and the overall shapes of the spectra
including the inter-absorption features show that the
carbonate in the crater best matches library spectra of
ankerite (Ca(Fe2+,Mg)(CO3)2), siderite (FeCO3), and/or
(Mg6Fe3+(CO3)(OH)13!4H2O) [26].
The shape of the Eridania carbonate spectra differs
from library spectra in that the 2.5µm is shallow relative to the 2.3µm absorption, and the “hump” between
them is shifted towards longer wavelengths. The
2.3µm minima are usually in the same location as
those in the library spectra, however, the top of the
2.3µm absorption widens towards the longer wavelengths.
Some of the carbonate spectra display a relatively
shallow slope from ~0.8µm to ~1.88µm. In continuum
removal, this feature appears bowl-shaped and has
relatively the same depth as the longer wavelength
features. The position of this feature is reasonably
matched with ankerite or siderite. Alternatively, it
could be due to a mixture of carbonates and Fesilicates, as has been interpreted elsewhere on Mars
Spectral Linear Mixing: A mixture between multiple minerals [e.g., 27] within the ROIs may be causing the shallow 2.5µm and the shifted “hump” feature.
Here we present linear mixtures between library Ankerite KACB01A and endmember minerals from the
entire CRISM library [25] as well as hydrous carbonates [26]. We find that the CRISM spectra can be
matched by mixtures of ankerite with small volumes of
pumpellyite, chamosite, chlorite, vermiculite, kaolinite-serpentine, and augite (Fig. 2). Also, these mixtures
replicate the shallow 1.0µm feature that is apparent in
the ROI spectra. No hydrous carbonate mixtures with
ankerite were found to produce analogous spectra. Although olivine has been detected with other carbonates
on Mars [4], mixtures between library olivine and ankerite did not match our spectra.
Implications: Terrestrial formation mechanisms
and locations where ankerite and one or more of the
silicate minerals can coexist are numerous. The most
Mars-relevant environments include diagenetic systems within sandstones and shales [28] at temperatures
from ~117 to ~200ºC, in geothermal regimes in siltstones and sandstones with brines containing 250,000
ppm dissolved solids and at temperatures between 80
and ~360ºC [29], and within hydrothermally altered
oceanitic basalts [30]. The minerals can also be found
together in serpentinites [31] at temperatures less than
an estimated 50ºC, as alteration halos around basaltic
dykes within dolomitic marbles [32], and potentially in
conjunction with CO2-rich brines at 95ºC and 10mPa
[33]. Thus, it seems that the minerals favor low to
moderate temperature regimes possibly akin to the
Figure 2: Linear mixing results between library [25] ankerite and
silicate minerals. The North Wall carbonate spectrum is in red.
low-grade metamorphism in the Mordenite to PrehnitePumpellyite facies [34] or as a result of diagenetic processes [28]. Water is also a major factor in these scenarios and acts predominantly as a weathering agent
[e.g., 28-30].
Conclusions: Carbonates have been identified
within an unnamed crater in the Eridania Basin. The
spectra are best matched by linear mixtures dominated
by ankerite (or potentially siderite, brugnatellite, and
aragonite) mixed with modest amounts of tectosilicates, phyllosilicates, or pyroxenes. On Earth, these
mineral assemblages form together in low to moderate
temperature hydrothermal systems. Therefore, the
Eridania carbonates may have formed under similar
circumstances as those that have been previously detected on Mars [4,5].
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